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Improving a Geophone to Produce an Affordable, Broadband Seismometer

Aaron Barzilai

PhD DefenseMechanical Engineering

Stanford University

January 25, 2000

2Aaron Barzilai Stanford University

Talk Outline

• Introduction

• Why isn’t a geophone an affordable, broadband seismometer?• How can a geophone be transformed into an affordable,

broadband seismometer? • How well do prototypes of a modified geophone perform?


The Primary Application

• A seismometer is a sensor that measures ground motion• PEPP (Princeton Earth Physics Project) is a group of

geophysics professors interested in placing seismometers in high school science classrooms

• Most areas don’t have local seismicity, so the only interesting signals are caused by teleseisms – large, distant earthquakes

Recorded at Stanford with a Guralp CMG-40T after a M7.3 Quake near Australia on 11/26/99

10 min


Teleseism Detection• Only low frequency (below 100 mHz-1 Hz) signals can

travel large distances

Requirements according to PEPP• Measure signals with periods as large as 30 sec

(frequencies as small as 33 mHz)• Constant sensitivity to ground velocity• Cost less than $1000

54 56 58 60 62 64 66-60-30

03060

Time [min]Gro

und

Vel

ocity

[um

/s]


Existing Commercial Seismometers

5 Hz –500 Hz

30 mHz –50 Hz

8 mHz –50 Hz

Constant Ground Velocity

Sensitivity

CostLow

Frequency Resolution

CommentType

$50PoorCheapestConventional

Geophone

$10,000GreatCompetitorGuralp

$20,000BestBestStreckeisen



30 mHz–50 Hz

$1,000DecentNewModified Geophone

5 Hz –500 Hz

30 mHz –50 Hz

8 mHz –50 Hz


Sensitivity

CostLow

Frequency Resolution

CommentType

$50PoorCheapestConventional

Geophone

$10,000GreatCompetitorGuralp

$20,000BestBestStreckeisen



30 mHz–50 Hz

$1,0005x10-6NewModified Geophone

5 Hz –500 Hz

30 mHz –50 Hz

8 mHz –50 Hz


Sensitivity

CostResolution at 30 mHz[(m/s)/√Hz]

CommentType

$502x10-4CheapestConventional

Geophone

$10,0008x10-8CompetitorGuralp

$20,0001x10-10BestStreckeisen


Introduction Recap

• Major application – Education• No commercially available seismometer delivers the

needed performance at a reasonable cost.• Plan to start with a geophone and improve it without

making cost unreasonable.


Talk Outline

• Introduction

• Why isn’t a geophone an affordable, broadband seismometer?

• How can a geophone be transformed into an affordable, broadband seismometer?

• How well do prototypes of a modified geophone perform?


What is a Conventional Geophone?


What is a Conventional Geophone?

CylinderCoil

Leaf SpringMagnet


How does a Conventional Geophone Work?

Cylinder

Coil

Leaf SpringMagnet





GroundMotion



MechanicalSystem

GroundMotion

RelativeMotion



MechanicalSystem

ElectricalSystem

GroundMotion

RelativeMotion

OutputSignal


Conventional Geophone Mechanical Sensitivity

MechanicalSystem

ElectricalSystem

kmb

mk

n

2=

=

ζ

ω22 2

1

nnssxy

ωζω ++=

&&Groundmotion

Relativemotion

Constant sensitivity below the resonant frequency

2nd Order System

10-6

10-5

10-4

10-3

10-2

10-2 10-1 100 101 102Mec

hani

cal S

ensi

tivity

[m/(

m/s

2 )]

Frequency [Hz]


100

101

102

103

104

105

10-2 10-1 100 101 102Ele

ctri

cal S

ensi

tivity

[V/m

]

Frequency [Hz]

Conventional Geophone Electrical Sensitivity

MechanicalSystem

ElectricalSystem

Reduced sensitivity at lower frequencies because measuring relative velocity

Faraday’s Law

yGt

V &=∂∂−= φ

Relativemotion

OutputVoltage Transduction Constant

[V/(m/s)] or [N/A]


Conventional Geophone Total Sensitivity

10-4

10-3

10-2

10-1

100

101

102

10-2 10-1 100 101 102Tot

al S

ensi

tivity

[V/(

m/s

)]

Frequency [Hz]

10-4

10-3

10-2

10-1

100

101

102

10-2 10-1 100 101 102Tot

al S

ensi

tivity

[V/(

m/s

2 )]

Frequency [Hz]

Electrical system measures velocity

Mechanical system has resonance


Conventional Geophone Resolution

Noise Sources• Thermomechanical noise• Circuitry noise

Geophone at low frequencies: Sensitivity ⇓, Resolution ⇑

][ySensitivit][ Noise

][ Resolution)/(

HzHz

)/(

smV

Vsm =

Want resolution to be a small number



Usher et al., “A miniature wideband horizontal component feedback seismometer,” Background Signals,” Journal of Physics E:Scientific Instruments, Dec. 1977, vol.10, no.12

( )

Hz)/(

10

min

Hz)/(10

min

min

2105.5

105.5

mTb4

2

sm

sm

b

fx

x

fkx

π

−

−

×=

×=

∆=

&

&&

&&

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-2 10-1 100 101 102

Geo

phon

e R

esol

utio

n [(

m/s

)/√H

z]

Frequency [Hz]

Thermo-mechanical

Limit



Rodgers, P.W., “Frequency Limits For Seismometers As Determined From Signal To Noise Ratios.Part 1. The Electromagnetic Seismometer,” BSSA, Apr. 1992, vol.82, no.2.

Geo100Ω

10kΩ

VO

Hz)/(

10

min 2105.5 sm

fx

π

−×=&

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-2 10-1 100 101 102

Geo

phon

e R

esol

utio

n [(

m/s

)/√H

z]

Frequency [Hz]

PredictedCircuitry Limit

Thermo-mechanical

Limit



Barzilai et al.., “Technique for measurement of the noise of a sensor in the presence of large background signals,” Review of Scientific Instruments, July 1998, vol.69, no.7.

Geo100Ω

10kΩ

VO

Hz)/(

10

min 2105.5 sm

fx

π

−×=&

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-2 10-1 100 101 102

Geo

phon

e R

esol

utio

n [(

m/s

)/√H

z]

Frequency [Hz]

PredictedCircuitry Limit

MeasuredResolution

Thermo-mechanical

Limit


Talk Outline• Introduction• Why isn’t a geophone an affordable, broadband seismometer?

– What is a geophone?– How does a geophone work?– What is the sensitivity of a geophone?– What is the resolution of a geophone?

– How was the geophone’s resolution measured?




Measuring a Conventional Geophone’s Resolution

Typical Experiment

10-9

10-8

10-7

10-6

10-5

10-1 100 101 102Gro

und

Vel

ocity

[(m

/s)/√

Hz]

Frequency [Hz]


Measuring a Conventional Geophone’s Resolution

Better Experiment

10-9

10-8

10-7

10-6

10-5

10-1 100 101 102

Frequency [Hz]

Geophone AGeophone B

Gro

und

Vel

ocity

[(m

/s)/√

Hz]


Determining Resolution from “Identical” Outputs



Coherence – – the fraction of the power of signal X that also appears in signal Y

Bendat and Piersol, Engineering Applications of Correlation and Spectral Analysis , 2nd ed. (Wiley, New York, 1993)

2XYγ



Coherence – – the fraction of the power of signal X that also appears in signal Y

( )YX

XYXY PSDPSD

CSD 22 )( =ωγ

Coherence

Bendat and Piersol, Engineering Applications of Correlation and Spectral Analysis , 2nd ed. (Wiley, New York, 1993)

2XYγ

Power SpectralDensity ∑

=

=dn

kk

dX T

TnG

1

2),(X2

)( ωω

Cross SpectralDensity ∑

=

=dn

kdXY TT

TnG

1k

*k ),(Y),(X2)( ωωω

)( Hz)/( 2sm



22

)()(

)()(

21

1)(

++

=

ωω

ωω

ωγ

U

N

U

N

XY

GG

GG

1)()(

)()(

21 ==

=

ωω

ωω

HH

GG NM

SeismicSignal

N

M

UX

Y

)(1 ωH

)(2 ωH

SensorNoise

TransferFunction

Output

+

+

+

+



22

)()(

)()(

21

1)(

++

=

ωω

ωω

ωγ

U

N

U

N

XY

GG

GG

)()()( ωωω UNX GGG +=

( ))(1)()(

)(1)()(2

2

ωγωω

ωγωω

XYXN

XYXN

NSDNSD

GG

−=

−=

Hz)(

Hz)( 2

sm

sm

SeismicSignal

N

M

UX

Y

)(1 ωH

)(2 ωH

SensorNoise

TransferFunction

Output

+

+

+

+



0

0.2

0.4

0.6

0.8

1

10-1 100 101 102

Coh

eren

ce []

Frequency [Hz]

10-10

10-9

10-8

10-7

10-6

10-5

10-1 100 101 102Gro

und

Vel

ocity

[(m

/s)/√

Hz]

Frequency [Hz]

Geophone AGeophone B

GeophoneResolution

2XYγ


Conventional Geophone Recap

• A conventional geophone has poor sensitivity at low frequencies because it inductively measures proof mass velocity

• The geophone’s poor low frequency sensitivity leads to poor low frequency resolution

• A technique has been demonstrated to measure the resolution of a seismometer in a noisy environment


Talk Outline

• Introduction• Why isn’t a geophone an affordable, broadband seismometer?




How Can We Improve a Geophone?

Rather than measure proof mass velocity,

100101102103104105106107

10-2 10-1 100 101 102Ele

ctri

cal S

ensi

tivity

[V/m

]

Frequency [Hz]


How Can We Improve a Geophone?

Rather than measure proof mass velocity, measure proof mass displacement

Have obtained the best results measuringproof mass displacement capacitively

100101102103104105106107

10-2 10-1 100 101 102Ele

ctri

cal S

ensi

tivity

[V/m

]

Frequency [Hz]


Capacitive Geophone Mechanical Hardware

Moving Electrode

Fixed Electrodes

Fixed Electrodes


Capacitance Electrical System

Modulate Measure

Demodulator

Demodulate

VPOSN-1

Position Measurement



Modulate Measure

Demodulator

Demodulate

VPOSN

Controller DriveCircuitry


-1

Feedback



Modulate Measure

Demodulator

Demodulate

VPOSN


SignalProcessing

VelocityOutput

Acceleration Output

Feedback


-1


Capacitive Geophone Control Design

totalx&&

)( 2sm

Mechanical System

1200421

2 ++ ss

y

(m)

5106×posnV

(V)

CapacitivePosition Sensing

( )sKcontV

(V)

Controller

3741contI

(A)

Resistor

30contF

(N)

Coil-Magnetas Actuator(G)

025.1

Mass

inx&&

)( 2sm

Drive Circuitry

contx&&

)( 2sm

+

-


Capacitive Geophone Control Design

• Controller aims to keep capacitances equal

• Design controller to produce correct sensitivity transfer function– Apply forces to counteract gravity– Apply forces to counteract seismic input

• Ensure system is stable– Loop gain must be less than 1 when phase dips below -180 degrees– Loop gain: Product of the transfer functions of all blocks of the system


10-3 10-2 10-1 100 101 102 103-200

-150

-100

-50

0

50

Frequency [Hz]

Loop

Gai

nPh

ase

[deg

]

10-3 10-2 10-1 100 101 102 10310-210-1100101102103104

Frequency [Hz]

Loo

p G

ain

Mag

nitu

de []

Loop Gain With Unity Gain Controller

-180

Gain>1


Inner Controller: Low Pass Filter

inx&& totalx&&

)( 2sm

Mechanical System

1200421

2 ++ ss

y

(m)

contx&&

)( 2sm

5106×posnV

(V)


125001

+s

contV

(V)

Controller

3741contI

(A)

Resistor

30contF

(N)Coil-Magnet

as Actuator(G)

025.1

Mass

2.5 M1000 µF

374

G

+

-


10-3 10-2 10-1 100 101 102 103-300-250-200-150-100

-500

Frequency [Hz]

Loop

Gai

nPh

ase

[deg

]

10-3 10-2 10-1 100 101 102 10310-8

10-6

10-4

10-2

100

102

Frequency [Hz]

Loo

p G

ain

Mag

nitu

de []

Loop Gain with Low Pass Filter

-180

Gain <1


Transfer Function with Low Pass Filter

10-3 10-2 10-1 100 101 102 10310-2

10-1

100

101

102

103

Frequency [Hz]Tra

nsfe

r Fun

ctio

n

Mag

nitu

de [V

/(m

/s2 )]

10-3 10-2 10-1 100 101 102 103-200-150-100

-500

50100

Frequency [Hz]

Tra

nsfe

r Fun

ctio

nPh

ase

[deg

]


Outer Controller: Lead Circuit

inx&& totalx&&

)( 2sm

Mechanical System

1200421

2 ++ ss

y

(m)

contx&&

)( 2sm

5106×posnV

(V)


125001

+s

contV

(V)

Controller

3741contI

(A)

Resistor

30contF

(N)Coil-Magnet

as Actuator(G)

025.1

Mass

contx&&

)( 2sm 1

121

22

11

1

2

++

sCRsCR

RRcontV

(V)3741contI

(A)30

contF

(N)025.1

+

- -


Lead Circuit Transfer Function

10-3 10-2 10-1 100 101 102 10310-2

10-1

100

Frequency [Hz]

Lea

d C

ircui

tM

agni

tude

[]

10-3 10-2 10-1 100 101 102 103-100

102030405060

Frequency [Hz]

Lea

d C

ircu

itPh

ase

[deg

]


10-3 10-2 10-1 100 101 102 10310-2

10-1

100

101

102

Frequency [Hz]

Loo

p G

ain

Mag

nitu

de []

10-3 10-2 10-1 100 101 102 103-200-150-100

-500

50100

Frequency [Hz]

Loop

Gai

nPh

ase

[deg

]Loop Gain with Full Controller

Phase > -180

Gain =1


Transfer Function with Full Controller

10-3 10-2 10-1 100 101 102 10310-2

10-1

100

101

Frequency [Hz]Tra

nsfe

r Fun

ctio

n

Mag

nitu

de [V

/(m

/s2 )]

10-3 10-2 10-1 100 101 102 103-200-150-100

-500

50100

Frequency [Hz]

Tra

nsfe

r Fun

ctio

nPh

ase

[deg

]



Modulate Measure

Demodulator

Demodulate

VPOSN


SignalProcessing

VelocityOutput

Acceleration Output

Feedback


-1


Capacitive Geophone Predicted Sensitivity

• Design predicts constant sensitivity to ground velocity from 10 mHz – 50 Hz

10-1

100

101

102

10-3 10-2 10-1 100 101 102

Sens

itivi

ty [V

/(m

/s)]

Frequency [Hz]


Modified Geophone Design Recap

• Capacitively measuring proof mass position improves the sensitivity, and therefore resolution, of a geophone

• A capacitive geophone operates with feedback to tune it’s frequency response

• Feedback forces are applied by running current through the geophone’s coil


Talk Outline

• Introduction• Why isn’t a geophone an affordable, broadband seismometer?• How can a geophone be transformed into an affordable,

broadband seismometer?

• How well do prototypes of a capacitive geophone perform?


Capacitive Geophone Sensitivity

• Measurement roughly, but not exactly, matches prediction

• Results obtained by measuring ambient seismic signal with both Capacitive Geophone and Guralp

• Guralp’s measurement not quite accurate over 50 Hz10-1

100

101

102

10-3 10-2 10-1 100 101 102

Sens

itivi

ty [V

/(m

/s)]

Frequency [Hz]


Measured Ambient Seismic Signals

• Above 0.3 Hz, seismometers have virtually identical outputs

• Below 0.3 Hz, capacitive geophone noise is greater than the ambient seismic signal, except peak at 7 seconds caused by the tides.

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-3 10-2 10-1 100 101

Frequency [Hz]

Guralp CMG-40T

Gro

und

Vel

ocity

[(m

/s)/√

Hz]

CapacitiveGeophone


Seismometer Resolution

• Used coherence to determine capacitive geophone resolution above 0.3 Hz

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-3 10-2 10-1 100 101

Frequency [Hz]

Res

olut

ion

[(m

/s)/√

Hz]

CapacitiveGeophone




• Capacitive Geophone has better resolution than a Conventional Geophone at low frequencies, but not at high frequencies

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-3 10-2 10-1 100 101

Frequency [Hz]

Res

olut

ion

[(m

/s)/√

Hz] Conventional

Geophone

CapacitiveGeophone





• Capacitive geophone is almost 2 orders of magnitude noisier than a Guralp CMG-40T

• Expect it can be improved…

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-3 10-2 10-1 100 101

Frequency [Hz]

Guralp CMG-40T

Res

olut

ion

[(m

/s)/√

Hz]

GeophoneThermo-

mechanicalLimit

ConventionalGeophone

CapacitiveGeophone


Seismic Vault


Seismic Vault


Seismic Vault: Uncovered


10-8

10-7

10-6

10-5

10-4

10-3 10-2 10-1 100 101

Frequency [Hz]

Res

olut

ion

[(m

/s)/√

Hz]

In Air

Inside"SeismicVault"

Environmental Coupling

30x10-6

6x10-6• At 30 mHz, seismic noise

reduces vault by a factor of 5

• Proves that outside seismic vault, low frequency resolution set by environmental noise

• Expect that is still true inside

• Expect vault to reduce temperature fluctuations and air currents


More on Environmental Coupling“…the sensor must be protected from variations of the atmospheric air pressure; its seismic mass would otherwise experience a variable buoyant force at least three orders of magnitude larger than the seismic background noise…” – Streckeisen †

† Streckeisen et al., “The Leaf-Spring Seismometer: Design and Performance,”Bull. Seismo. Soc. of America, vol.72, no.6, p.2352, December 1982.

gVF proofbuoy ρ= gVRTPW

F proofbuoy =

• Buoyancy force varies due to air density variations• Ideal gas law predicts air density varies with pressure


0 20 40 60 80 100 120-20-10

01020

Time [min]

Pres

sure

Var

iatio

n [P

a]Expected Pressure Variation

0 20 40 60 80 100 120-10-505

10

Time [min]

Gro

und

Vel

ocity

[um

/s]

Capacitive Geophone Guralp CMG-40T


Measured Atmospheric Pressure Variations

• Data obtained from University of Washington, Dept of Atmospheric Sciences, J. E. Tillman and Neal C. Johnson (http://www.atmos.washington.edu/~neallog/temp_real_pressure.html)

• 1 mbar = 100 Pa


What other changes are possible?

• Electron Tunneling for displacement sensing– Tunneling is a quantum mechanical effect that occurs when two

electrodes with a voltage potential between them come very closetogether

– Typically see 1nA currents when electrodes are about 10 Å apart– Current varies with gap, yielding a generic motion sensing

mechanism, similar to capacitance or inductance– Has been used in infrared sensors and accelerometers

• Velocity Feedback geophone– Use coil as both sensor and actuator– Only add circuitry, no mechanical hardware modifications


Tunneling Currenty

oeII φα−=

0

5

10

15

20

0 20 40 60 80 100

Tun

nelin

g C

urre

nt[n

A]

Electrode Gap [Angstroms]

OperatingPoint

Hz13-10 of Resolution Typical m


Tunneling Implemented

Fixed Mount

Moving Mount

MovingElectrode

Fixed Electrode

EpoxySupport


Tunneling Results

• Obtained expected sensitivity of 2.5x103 V/(m/s2) in practice when operating correctly

• Difficult to hold electrode gap steady– Sensitive to the environment as well as ground motion

• Attempted to measure work function F– Could only set bounds on the value– Greater than 10-2eV, impossible to be over 1eV


Tunneling Resolution

• Tunneling geophone sees the peak at 35 Hz

• In general, tunneling geophone’s resolution is poor

• Believe it is measuring signals due to other stimuli, such as air currents

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1 100 101 102

Frequency [Hz]

Res

olut

ion

[(m

/s)/√

Hz]

TunnelingGeophone

Guralp CMG-40T


Velocity Feedback Control Design

totalx&&

)( 2sm

Mechanical System

1200421

2 ++ ss

y

(m)Gs

posnV

(V)

InductiveVelocity Sensor

( )sKcontV

(V)

Controller

3741contI

(A)

Resistor

GcontF

(N)

Coil-Magnetas Actuator(G)

025.1

Mass

inx&&

)( 2sm

Drive Circuitry

contx&&

)( 2sm

+

-


Velocity Feedback Measured Sensitivity

• Model predicts constant sensitivity to ground acceleration out to 50 Hz

• Measured data matches well

• This approach won’t improve low frequency resolution

• Velocity Feedback won’t produce an affordable, broadband seismometer 10-2

10-1

100

101

100 101 102 103

Frequency [Hz]

Sens

itivi

ty [V

/g]


Talk Outline

• Introduction• Why isn’t a geophone an affordable, broadband seismometer?• How can a geophone be transformed into an affordable,

broadband seismometer? • How well do prototypes of a capacitive geophone perform?

– Does the measured sensitivity match predictions?– Has the low frequency resolution been improved?– What limits the low frequency resolution?

– Do the prototypes meet PEPP’s requirements?


Measured Distant Earthquakes

M7.3 Quake near Australia on 11/26/99 recorded at Stanford

54 56 58 60 62 64 66-60

-30

0

30

60

Time [min]

Mea

sure

d G

roun

d M

otio

n [u

m/s

]

Guralp CMG-40TCapacitive Geophone


0 0.5 1 1.5 2-60-30

03060

Cap

Geo

O

utpu

t [um

/s]

0 0.5 1 1.5 2-60-30

03060

Gur

alp

Out

put [

um/s

]

0 0.5 1 1.5 2-300-200-100

0100

Time [hours]

Con

v G

eoO

utpu

t [m

V]


Cost Estimate: Volumes ≈10

From Standard suppliers (eg Digikey, Allied)$65PCB Components

8 hours at Grad Student Wages$68Labor

Meets Target$453Total

Paramont Precision Inc$150Current Mechanical Hardware

AP Circuits$20Printed Circuit Board

Paramont Precision Inc$85Environmental Isolation Mechanical Hardware

Includes setup fee, OYO Geospace $65Geophone with holes

CommentCostItem


Conclusions and Contributions

• Have modified a geophone to produce a seismometer capable of measuring teleseisms

• Expected price could be less than $1,000, enabling their use in high schools

• Presented a technique for measuring noise of sensors in the presence of large background signals

• Next step: In conjunction with PEPP, develop a more professional prototype with better environmental isolation


Publications• A. Barzilai, T. VanZandt, and T. Kenny, ``Technique for measurement of the noise of a sensor in the presence of

large background signals,'' Review of Scientific Instruments, vol. 69, no. 7, pp. 2767-2772, July 1998.• C.H. Liu, A. Barzilai, J.K. Reynolds, A. Partridge, T. Kenny, J. Grade, and H. Rockstad, ``Characterization of a

high-sensitivity micromachined tunneling accelerometer with micro-g resolution,'‘ Journal of Microelectromechanical Systems, vol. 7, no. 2, pp. 235-244, July 1998.

• A. Barzilai, T. VanZandt, T. Pike, S. Manion, and T. Kenny, ``Tunneling Seismometers: A Tunneling Geophone,'' Conference Proceedings, American Society of Mechanical Engineers International Congress, Nov. 1999.

• C.H. Liu, A. Barzilai, O. Ajakaiye, H.K. Rockstad, and T.W. Kenny, ``Performance Enhancements for theMicromachined Tunneling Accelerometer", Conference Proceedings, International Conference on Solid State Sensors and Actuators, June 1999.

• A. Barzilai, T. VanZandt, T. Pike, S. Manion, and T. Kenny, ``Improving the Performance of a Geophone through Capacitive Position Sensing and Feedback,'' Conference Proceedings, American Society of Mechanical Engineers International Congress, Nov. 1998.

• C.H. Liu, J.K. Reynolds, A. Partridge, J. Grade, A. Barzilai, T. Kenny, and H. Rockstad, ``High-sensitivity micromachined accelerometer based on electron tunneling transducers,'' Conference Proceedings, American Society of Mechanical Engineers International Congress, Nov. 1997.

• J. Grade, A. Barzilai, J.K. Reynolds, C.H. Liu, A. Partridge, H. Jerman, and T. Kenny, ``Wafer-scale processing, assembly, and testing of tunneling infrared detectors'' Conference Proceedings, International Conference on Solid-State Sensors and Actuators, June 1997.

• J. Grade, A. Barzilai, J.K. Reynolds, C.H. Liu, A.Partridge, L. Miller, J. Podosek, and T. Kenny, ``Low frequency drift in tunnel sensors'' Conference Proceedings, International Conference on Solid-State Sensors and Actuators, June 1997.

• C.H. Liu, J. Grade, A. Barzilai, J.K. Reynolds, A.Partridge, H. Rockstad, and T. Kenny, ``Characterization of a high-sensitivity micromachined tunneling accelerometer'' Conference Proceedings, International Conference on Solid-State Sensors and Actuators, June 1997.

• J. Grade, A. Barzilai, J.K. Reynolds, C.H. Liu, A.Partridge, T. Kenny, T.VanZandt, L. Miller, and J. Podosek, ``Progress in tunnel sensors'‘ Conference Proceedings, Solid-State Sensor and Actuator Workshop, June 1996.


Acknowledgements• Tom Kenny• Ed Carryer, Chris Gerdes, Steve Rock, Greg Beroza• Tom VanZandt, Steve Manion, Tom Pike @ JPL (CSMT)• NSF Career Award, Charles Lee Powell Foundation,

Terman Fellowship• IRIS(Marcos Alvarez), PSN(Larry Cochrane), PEPP,

SPDL, PRL, CDR, RPL, Goodson Lab, Biomotion Lab, Design Division

• Kennylab, particularly John Grade, Jonah Harley, Cheng-Hsien Liu, Olaleye Ajakaiye, Kevin Yasumura, Eugene Chow

• Friends on and off campus• Family


Acknowledgements

Thanks, Jess



Inner Controller Transfer Function

10-5 10-4 10-3 10-2 10-1 100 101 102 10310-8

10-6

10-4

10-2

100

Frequency [Hz]

Con

trol

ler

Mag

nitu

de []

10-5 10-4 10-3 10-2 10-1 100 101 102 103-100

-80

-60

-40

-20

0

Frequency [Hz]

Con

trol

ler

Phas

e [d

eg]


Outer Controller: Circuit

2.5 M

0.1 µF

G

10 k

10 k 47.5 k

0.1 µF

4.99 k

10 k 10 k

1000 µF

10 k

10 k

374


Outer Controller: Extend Bandwidth

inx&& totalx&&

)( 2sm

Mechanical System

1200421

2 ++ ss

y

(m)

contx&&

)( 2sm

5106×posnV

(V)


125001

+s

contV

(V)

Controller

3741contI

(A)

Resistor

30contF

(N)Coil-Magnetas Actuator

025.1

Mass

contx&&

)( 2sm 1

121

22

11

1

2

++

sCRsCR

RRcontV

(V)3741contI

(A)30

contF

(N)025.1


Position Measurement Circuitry

A

4

2

5

3

B

10 M

TLC2274

10 k

10 k

OP470

MAX4526

2.21 k

0.1 µF

OP470

A

B

C

C

4

2




• Capacitive geophone is almost 2 orders of magnitude noisier than a Guralp CMG-40T

• Expect Capacitive Geophone noise can be reduced by reducing coupling to the environment….

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-3 10-2 10-1 100 101

Frequency [Hz]

Guralp CMG-40T

Res

olut

ion

[(m

/s)/√

Hz]

GeophoneThermomechanical

Limit

ConventionalGeophone

CapacitiveGeophone


More on Environmental Coupling

• According to Streckeisen, “…the sensor must be protected from variations of the atmospheric air pressure; its seismic mass would otherwise experience a variable buoyant force at least three orders of magnitude largerthan the seismic background noise…” †

† Streckeisen et al., “The Leaf-Spring Seismometer: Design and Performance,”Bull. Seismo. Soc. of America, vol.72, no.6, p.2352, December 1982.

MovingMass

mg ρVg

ky Buoyancy Force ( )?tρ

VRT

WmP =

Wmn =nRTPV =

WRTP ρ=

RTPW=ρ



gVF proofbuoy ρ= gVRTPW

F proofbuoy =

KmolJ

molkg

air .RW ⋅− =×= 3148 1097.28 3

KT.gsm 300 89 2 ≈=

( )( )322223 108.5]1065.1[]102.2[]m[ −−− ××−×= πproofV.85 in .65 in .2 in

]m[109.3 36−×=proofV

AssumeConstant

]m[104.4][]N[ 210mN

2−×⋅= PFbuoy



][108.1][ 22 mkgN8

mN

⋅−×⋅= Pxbuoy&&

][108.1][21

22 mkgN8

mN

⋅−×⋅= P

fxbuoy π&


Power Consumption

• Capacitive Geophone: – +7V, 27mA & -7V, 21mA ⇒ 330 mW total– 8mA each for 2 OP-470’s = 16mA– 3 mA for TLC2274– 0.5 mA each for 2 MAX4526’s = 1mA– 0.5 µA for HA7210 =0.0005 mA

• Streckeisen– 700 mW for 3-axis “Low Power” seismometer

• Guralp– 550 mW for 3-axis seismometer


Placing Geophone Mechanics Outside Seismic Vault Causes Most Noise

• Noise is largest when geophone mechanics are outside vault, electronics in

• Slightly increased noise if electronics outside vault, mechanics inside

• Have both inside as a reference• Data is repeatable• One electronics outside result

obtained after testing mechanics outside, 2 others taken before

10 -8

10 -7

10 -6

10 -5

10 -4

10 -3

10 -2 10 -1 10 0Vel

ocity

NSD

[(m

/s)/¦

Hz]

Frequency [Hz]

Datasets

Green: Mech Inside, Elec InsideBlue: Mech Inside, Elec Outside

9,10,11,1207,08,13

05

Red: Mech Outside, Elec Inside

Taken From Report MechEnvSens110999


Schematic

AdditionalHousing

MovingElectrode

FixedElectrodes

Insulation

33.37 mm39.37 mm

y

CircuitModel

a = Balanced Gap ≈ 250µm

A = Area = 3.4 ×10-4 m2

CNOMINAL = 12.1pF

C =εε0 Aa − y

C =εε0 Aa + y

Documents

Improving a Geophone to Produce an Affordable, Broadband ...micromachine.stanford.edu/projects/geophones/DefenseBarzilaiFinal... · Improving a Geophone to Produce an Affordable,